The present invention relates to image-forming optical systems. More particularly, the present invention relates to a decentered optical system with a reflecting surface having a power for use in optical apparatus using a small-sized image pickup device, e.g. video cameras, digital still cameras, film scanners, and endoscopes.
Recently, with the achievement of small-sized image pickup devices, image-forming optical systems for use in video cameras, digital still cameras, film scanners, endoscopes, etc. have also been demanded to be reduced in size and weight and also in cost.
In the general rotationally symmetric coaxial optical systems, however, optical elements are arranged in the direction of the optical axis. Therefore, there is a limit to the reduction in thickness of the optical systems. At the same time, the number of lens elements unavoidably increases because it is necessary to correct chromatic aberration produced by a rotationally symmetric refracting lens used in the optical systems. Therefore, it is difficult to reduce the cost in the present state of the art. Under these circumstances, there have recently been proposed optical systems designed to be compact in size by giving a power to a reflecting surface, which produces no chromatic aberration, and folding an optical path in the optical axis direction.
Japanese Patent Application Unexamined Publication (KOKAI) Number [hereinafter referred to as “JP(A)”] 7-333505 proposes to reduce the thickness of an optical system by giving a power to a decentered reflecting surface and folding an optical path. In an example thereof, however, the number of constituent optical members is as large as five, and actual optical performance is unclear. No mention is made of the configuration of the reflecting surface.
JP(A) 8-292371, 9-5650 and 9-90229 each disclose an optical system in which an optical path is folded by a single prism or a plurality of mirrors integrated into a single block, and an image is relayed in the optical system to form a final image. In these conventional examples, however, the number of reflections increases because the image is relayed. Accordingly, surface accuracy errors and decentration accuracy errors are transferred while being added up. Consequently, the accuracy required for each surface becomes tight, causing the cost to increase unfavorably. The relay of the image also causes the overall volumetric capacity of the optical system to increase unfavorably.
JP(A) 9-222563 discloses an example of an optical system that uses a plurality of prisms. However, because the optical system is arranged to relay an image, the cost increases and the optical system becomes large in size unfavorably for the same reasons as stated above.
JP(A) 9-211331 discloses an example of an optical system in which an optical path is folded by using a single prism to achieve a reduction in size of the optical system. However, the optical system is not satisfactorily corrected for aberrations.
JP(A) 10-68887 discloses an example of an optical system capable of image formation without relaying an image by using two prism optical systems. The optical system has a roof surface for erecting an image formed by an objective lens, and this is a design example having a very narrow field angle. In a decentered prism optical system, it is difficult to predict ghost light (stray light). Therefore, it is necessary to minimize the occurrence of ghost light during the design phase. In a particular, in a case where the surface closest to the image plane side is mutually used as a totally reflecting surface and an exit surface as in JP(A) 10-68887, when light rays enter the optical system at somewhat different incident angles, there is a possibility of light passing through the totally reflecting surface to leak into the image pickup surface as ghost light or flare.
Similarly, when a prism is placed closer to the object side than an aperture stop, because the height of extra-axial rays is high, the effective surface area of the prism is inevitably large. Accordingly, ghost light is likely to occur.
When a general refracting optical system is used to obtain a desired refracting power, chromatic aberration occurs at an interface surface thereof according to chromatic dispersion characteristics of an optical element. To correct the chromatic aberration and also correct other ray aberrations, the refracting optical system needs a large number of constituent elements, causing the cost to increase. In addition, because the optical path extends straight along the optical axis, the entire optical system undesirably lengthens in the direction of the optical axis, resulting in an unfavorably large-sized image pickup apparatus.
In decentered optical systems such as those described above in regard to the prior art, an imaged figure or the like is undesirably distorted and the correct shape cannot be reproduced unless the formed image is favorably corrected for aberrations, particularly rotationally asymmetric distortion.
Furthermore, in a case where a reflecting surface is used in a decentered optical system, the sensitivity to decentration errors of the reflecting surface is twice as high as that in the case of a refracting surface, and as the number of reflections increases, decentration errors increase correspondingly because they are transferred while being added up. Consequently, manufacturing accuracy and assembly accuracy, e.g. surface accuracy and decentration accuracy, required for reflecting surfaces become even more strict.
With the recent development of manufacturing techniques, solid-state image pickup devices are reducing in size and becoming higher in definition, year by year. In the present state of the art, the size of one pixel is of the order of several micrometers. In image pickup devices, e.g. CCDs, currently on the digital camera market in particular, those with megapixels are becoming the mainstream. Because such image pickup devices are expected to become even more denser in future, lenses of higher performance will be needed. Thus, it is becoming difficult to ensure the required performance with only one prism.